1. Field of the Invention
This invention relates to a display device using a light emitting element and a driving method thereof. More particularly, it relates to an active matrix type display device in which a light emitting element is disposed in each pixel and a transistor for controlling light emission of the light emitting element is provided and a driving method thereof.
2. Description of the Related Art
Development of a display device having a light emitting element has been put forward in these years. In particular, development of an active matrix type display device in which a light emitting element and a transistor for controlling light emission of the light emitting element are disposed in each pixel has been put forward.
In the active matrix type display device, either a technique in which an input of luminance information to each pixel is carried out by a voltage signal or a technique in which it is carried out by a current signal is mainly used. The former is called as a voltage writing type, and the latter is called as a current writing type. These structures and driving methods will be, hereinafter, described in detail.
Firstly, one example of a pixel of the voltage writing type is shown in FIG. 26 and its structure and driving method will be described. In each pixel, two TFT(a switching TFT 3001 and a driving TFT 3004) and a holding capacitor 3007 and an EL element 3006 are disposed. Here, a first electrode 3006a of the EL element 3006 is called as a pixel electrode, and a second electrode 3006b is called as a counter electrode.
A driving method of the above-described pixel will be described. When the switching TFT 3001 is turned ON by a signal which is inputted to a gate signal line 3002, an electric charge is stored and held in the holding capacitor 3007 by a voltage of a video signal which is inputted to a source signal line 3003. A current which amount corresponds to the electric charge held in the holding capacitor 3007 flows from a power supply line 3005 to the EL element 3006 through the driving TFT 3004 so that the EL element 3006 emits light.
In pixels of the voltage writing type, the video signal which is inputted to the source signal line 3003 may be of an analog system or may be of a digital system. Driving in a case that the analog system video signal was used is called as the analog system, and driving in a case that the digital system video signal is used is called as the digital system.
In the voltage writing type analog system, a gate voltage (a voltage between a gate and a source) of each pixel of the driving TFT 3004 is controlled by the analog video signal. And, by the drain current with a value comparable to the gate voltage flowing through the EL element 3006, luminance is controlled and gray scale is displayed. On this account, generally in the voltage writing type analog system, in order to display halftone gray scale level, the driving TFT 3004 is made to operate in such an area that change of the drain current is larger relative to that of the gate voltage.
On the other hand, in the voltage writing type digital system, periods of light emission is controlled and gray scales are displayed by selecting using digital signals whether the EL element 3006 is made to emit light or not. In short, the driving TFT 3004 takes a function as a switch. On this account, generally in the voltage writing type digital system, on the occasion that the EL element 3006 is made to emit light, the driving TFT 3004 is made to operate in a linear region, more closely, particularly an area in which an absolute value of the gate voltage is large in the linear region.
The operation area of the driving TFT in the voltage writing type digital system and the voltage writing type analog system will be described by use of FIGS. 27A and 27B. FIG. 27A is a view, for the purpose of simplicity, showing only the driving TFT 3004, the power supply line 3005 and the EL element 3006 in the pixel shown in FIG. 26. Curves 3101a and 3101b in FIG. 27B shows a value of the drain current Id corresponding to the gate voltage Vgs of the driving TFT 3004, respectively. The curve 3101b shows a characteristic in a case that a threshold voltage of the driving TFT 3004 is changed from that in case of the curve 3101a. 
In the voltage writing analog system, the driving TFT 3004 operates in an operation area shown by (1) in FIG. 27B. In the operation area (1), when a gate voltage Vgs1 is applied, if a current characteristic of the driving TFT 3004 varies from 3101a to 3101b, the drain current changes from Id1 to Id2. In short, in the voltage writing type analog system, when the current characteristic of the driving TFT 3004 varies, the drain current varies and therefore, there is a problem that luminance of the EL element 3006 varies among pixels.
On the other hand, the driving TFT in the voltage writing type digital system operates in an operation area shown by (2) in the figure. The operation area (2) corresponds to the linear region. The driving TFT 3004 which operates in the linear region, in case that the same gate voltage Vgs2 is applied, have substantially a constant current Id3 flown since variation of the drain current resulting from variation of the characteristic such as mobility and threshold voltage is small. Thus, in the voltage writing type digital system in which the driving TFT 3004 operates in the operation area (2), even if the current characteristic of the driving TFT 3004 varies from 3101a to 3101b, it is hard for the current flowing through the EL element 3006 to vary, and it is possible to suppress variation of light emission luminance.
Thus, it can be said that as to the variation of luminance of the EL element resulting from the variation of the current characteristic of the driving TFT 3004, that of the voltage writing type digital system is smaller than that of the voltage writing type analog system.
Then, a structure and a driving method of the pixel of the current writing type will be described.
In a display device of the current writing type, a current of the video signal (signal current) is inputted from the source signal line to each pixel. The signal current has a current value which linearly corresponds to luminance information. The signal current which was inputted becomes a drain current of a TFT included in a pixel. A gate voltage of the TFT is held in a capacitance part included in a pixel. Even after input of the signal current is terminated, the drain current of TFT is maintained to be constant by the held gate voltage, and by inputting the drain current to the EL element, the EL element emits light. In this manner, in the current writing type display device, a current flowing through the EL element is made to be changed by changing magnitude of the signal current so that the light emission luminance of the EL element is controlled and gray scale is displayed.
Hereinafter, two structures of the pixels of the current writing type are illustrated for example, and those structures and driving methods thereof will be described in detail.
FIG. 28 shows a structure of a pixel which is described in a patent document 1(JP-T-2002-517806) and a non patent document 1(IDW'00 p235-p238: Active Matrix PolyLED Displays). The pixel shown in FIG. 28 has an EL element 3306, a switching TFT 3301, a driving TFT 3303, a holding capacitor 3305, a holding TFT 3302, and a light emitting TFT 3304. Also, 3307 designates a source signal line, and 3308 designates a first gate signal line, and 3309 designates a second gate signal line, and 3310 designates a third gate signal line, and 3311 designates a power supply line. A current value of the signal current which is inputted to the source signal line 3307 is controlled by a video signal input current source 3312.
A driving method of the pixel of FIG. 28 will be described by use of FIGS. 29A to 29D. In addition, in FIGS. 29A to 29D, the switching TFT 3301, the holding TFT 3302 and the light emitting TFT 3304 are shown as switches.
In a period of TA1, the switching TFT 3301 and the holding TFT 3302 are turned ON. In this moment, the power supply line 3311 is connected to the source signal line 3307 through the driving TFT 3303 and the holding capacitor 3305. Through the source signal line 3307, a current amount Ivideo defined by a video signal input current source 3312 flows. Therefore, when time passes and it becomes a stable state, the drain current of the driving TFT 3303 becomes Ivideo. Also, the gate voltage corresponding to the drain current Ivideo is held in the holding capacitor 3305 (FIG. 29A). After the drain current of the driving TFT 3303 is settled to be Ivideo, a period of TA2 is initiated, and the holding TFT 3302 is turned OFF.
Next, a period of TA3 is initiated, the switching TFT 3301 is turned OFF (FIG. 29C). Further, in a period of TA4, when the light emitting TFT 3304 is turned ON, the signal current Ivideo is inputted from the power supply line 3311 to the EL element 3306 through the driving TFT 3303. By this means, the EL element 3306 emits light with luminance corresponding to the signal current Ivideo, In the pixel shown in FIG. 28, by analogously changing the signal current Ivideo, it is possible to express the gray scale.
In the above-described current writing type display device, the drain current of the driving TFT 3303 is determined by the signal current which is inputted from the source signal line 3307, and still further, the driving TFT 3303 operates in a saturation region. Therefore, even if there is variation of the characteristic of the driving TFT 3303, the gate voltage of the driving TFT 3303 automatically changes in such a manner that a constant drain current flows through the light emitting element. In this manner, in the current writing type display device, even if the characteristic of TFTs varies, it is possible to suppress variation of a current flowing through the EL element. As a result, it is possible to suppress the variation of the light emission luminance.
Next, another example of the current writing type pixel which is different from FIG. 28 will be described. FIG. 30A shows a pixel which is described in a patent document 2(JP-A-2001-147659).
A pixel shown in FIG. 30A is configured by an EL element 2906, a switching TFT 2901, a driving TFT 2903, a current TFT 2904, a holding capacitor 2905, a holding TFT 2902, a source signal line 2907, a first gate signal line 2908, a second gate signal line 2909,and a power supply line 2911. It is necessary for the driving TFT 2903 and the current TFT 2904 to have the same polarity. Here, for the purpose of simplicity, it is assumed that an Id-Vgs characteristic (a relation of the drain current and the voltage between gate and drain) of the driving TFT 2903 is the same as that of the current TFT 2904. Also, a current value of the signal current which is inputted to the source signal line 2907 is controlled by the video signal input current source 2912.
A driving method of the pixel shown in FIG. 30A will be described by use of FIGS. 30B to 30D. In addition, in FIGS. 30B to 30D, the switching TFT 2901 and the holding TFT 2902 are shown as switches.
In the period of TA1, when the switching TFT 2901 and the holding TFT 2902 are turned ON, the power supply line 2911 is connected to the source signal line 2907 through the current TFT 2904, the switching TFT 2901, the holding TFT 2902 and the holding capacitor 2905. Through the source signal line 2907, the current amount Ivideo which is defined by the video signal input current source 2912 flows. Therefore, when sufficient time passes and it becomes a stable state, the drain current of the current TFT 2904 becomes Ivideo, and the gate voltage corresponding to the drain current Ivideo is held in the holding capacitor 2905.
After the drain current of the current TFT 2904 is settled to be Ivideo, the period of TA2 is initiated, and the holding TFT 2902 is turned OFF. In this moment, through the driving TFT 2903, the drain current of Ivideo flows. In this manner, the signal current Ivideo is inputted from the power supply line 2911 to the EL element 2906 through the driving TFT 2903. The EL element 2906 emits light with luminance in response to the signal current Ivideo.
Next, when the period of TA3 is initiated, the switching TFT 2901 is turned OFF. Even after the switching TFT 2901 is turned OFF, the signal current Ivideo continues to be inputted from the power supply line 2911 to the EL element 2906 through the driving TFT 2903, and the EL element 2906 continues to emit light. The pixel shown in FIG. 30A can express gray scale by analogously changing the signal current Ivideo.
In the pixel shown in FIG. 30A, the driving TFT 2903 operates in the saturation region. The drain current of the driving TFT 2903 is determined by the signal current which is inputted from the source signal line 2907. Therefore, if the current characteristics of the driving TFT 2903 and the current TFT 2904 in the same pixel are equivalent, even if there is variation of the characteristics of the driving TFTs 2903, the gate voltage of the driving TFT 2903 automatically changes in such a manner that a constant drain current is made to flow through the light emitting element.
In the EL element, a relation of a voltage between both electrodes thereof and a flowing current amount (I-V characteristic) changes due to influence of ambient temperature, deterioration over time and so on. Therefore, in a display device in which the driving TFT is operated in the linear region like the above-described voltage writing type digital system, even if a voltage value between both electrodes of the EL element is the same, the current amount flowing between both electrodes of the EL element is changed.
In the voltage writing type digital system, FIGS. 31A and 31B are views showing a change of an operating point in a case that the I-V characteristic of the EL element is changed due to deterioration etc. In addition, in FIGS. 31A and 31B, same reference numerals are given to those portions which are the same as the corresponding portions of FIGS. 27A and 27B.
FIG. 31A is a view, for the purpose of simplicity, showing only the driving TFT 3004 and the EL element 3006 in FIG. 26. A voltage between a source and a drain of the driving TFT 3004 is represented by Vds. A voltage between both electrode of the EL element 3006 is shown by VEL. A current flowing through the EL element 3006 is shown by IEL. The current IEL equals to the drain current Id of the driving TFT 3004. An electric potential of the power supply line 3005 is shown by Vdd. Also, an electric potential of a counter electrode of the EL element 3006 is assumed to be 0(V).
In FIG. 31B, 3202a designates a curve which shows the relation of the voltage VEL and the current amount IEL of the EL element 3006 before deterioration (I-V characteristic). On the other hand, 3202b designates a curve which shows I-V characteristic of the EL element 3006 after deterioration. 3201 designates a curve which shows the relation of the voltage between source and drain Vds and the drain current Id(IEL) of the driving TFT 3004 in a case that the gate voltage in FIG. 27B is Vgs2. Operating conditions (operating points) of the driving TFT 3004 and the EL element 3006 are determined by an intersection point of these two curves. In short, by the intersection point 3203a of the curve 3202a and the curve 3201 in the linear region shown in the figure, the operating conditions of the driving TFT 3004 and the EL element 3006 before deterioration of the EL element 3006 are determined. Also, by the intersection point 3203b of the curve 3202b and the curve 3201 in the linear region shown in the figure, the operating conditions of the driving TFT 3004 and the EL element 3006 after deterioration of the EL element 3006 are determined. The operating points 3203a and 3203b will be compared to each other.
In the pixel which was selected to be in a light emitting state, the driving TFT 3004 is in a state of ON. In this moment, a voltage between both electrodes of the EL element 3006 is VA1. When the EL element 3006 is deteriorated and its I-V characteristic is changed, even if the voltage between both electrodes of the EL element 3006 is substantially the same as VA1, a flowing current is changed from IEL1 to IEL2. In short, since the current flowing through the EL element 3006 is changed from IEL1 to IEL2 by a level of deterioration of the EL element 3006 of each pixel, the light emission luminance is varied.
As a result, in a display device having a pixel of such a type that the driving TFT is made to be operated in the linear region, burn-in of an image tends to occur.
On the other hand, in the pixels of the current writing type shown in FIGS. 28 and 30A to 30D, the above-described burn-in of the image is reduced. This is because, in the pixel of the current writing type, the driving TFT operates so as to always flow substantially a constant current.
In the pixel of the current writing type, change of the operating point in a case that the I-V characteristic of the EL element, in the current writing type, is changed due to deterioration etc. will be described by use of the pixel of FIG. 28 as an example. FIG. 32 is a view showing the change of the operating point in the case that the I-V characteristic of the EL element is changed due to deterioration etc. In addition, in FIG. 32, same reference numerals are given to those portions which are the same as the corresponding portions of FIG. 28.
FIG. 32A is a view, for the purpose of simplicity, showing only the driving TFT 3303 and the EL element 3306 in FIG. 28. A voltage between a source and a drain of the driving TFT 3303 is shown by Vds. A voltage between a cathode and an anode of the EL element 3306 is shown by VEL. A current flowing through the EL element 3306 is shown by IEL. The current IEL equals to the drain current Id of the driving TFT 3303. An electric potential of the power supply line 3005 is shown by Vdd. Also, an electric potential of a counter electrode of the EL element 3306 is assumed to be 0(V).
In FIG. 32B, 3701 designates a curve which shows the relation of the voltage between source and drain and the drain current of the driving TFT 3303. 3702a designates a curve which shows the I-V characteristic of the EL element 3306 before deterioration. On the other hand, 3702b designates a curve which shows the I-V characteristic of the EL element 3306 after deterioration. Operating conditions of the driving TFT 3004 and the EL element 3006 before deterioration of the EL element 3306 are determined by an intersection point 3703a of the curves 3702a and 3701. Operating conditions of the driving TFT 3303 and the EL element 3306 after deterioration of the EL element 3306 are determined by an intersection point 3703b of the curves 3702b and 3701. Here, the operating points 3703a and 3703b will be compared to each other.
In the pixel of the current writing type, the driving TFT 3303 operates in the saturation region. Before and after the EL element 3006 is deteriorated, the voltage between both electrodes of the EL element 3006 is changed from VB1 to VB2 but, the current flowing through the EL element 3006 is maintained to be IEL1 which is substantially constant. In this manner, even if the EL element 3006 is deteriorated, the current flowing through the EL element 3006 is maintained to be substantially constant. Thus, the problem of the burn-in of the image is reduced.
However, in the conventional driving method of the current writing type, there is a necessity that electric charge corresponding to the signal current is held in the holding capacity of each pixel. The operation for holding a predetermined electric charge in the holding capacitor needs longer time as the signal current becomes smaller, because of an intersection capacitance etc. of a wiring through which the signal current flows. On that account, it is difficult to quickly write the signal current. Also, in case that the signal current is small, large is influence of a noise of a leak current etc. which occurs from a plurality of pixels connected to the same source signal line as that of the pixel to which writing of the signal current is carried out. Therefore, there is such a high risk that it is impossible for the pixel to emit light with accurate luminance.
Also, in the pixel having a current mirror circuit represented by the pixel shown in FIG. 30, it is desirable that a pair of TFTs which configure the current mirror circuit have same current characteristics. However, in reality, it is hard that the pair of these TFTs have completely the same current characteristics, and there occurs variation.
In the pixel shown in FIG. 30, threshold values of the driving TFT 2903 and the current TFT 2904 are Vtha, Vthb, respectively. When the threshold values Vtha, Vthb of both transistors vary and an absolute value |Vtha| of Vtha has become smaller than an absolute value |Vthb| of Vthb, a case of carrying out a black display will be considered. The drain current flowing through the current TFT 2903 is comparable to the current value Ivideo determined by the video signal input current source 2912, and assumed to be 0. However, even if the drain current does not flow through the current TFT 2904, there is a possibility that a voltage of a level of slightly smaller than |Vthb| is held in the holding capacitor 2905. Here, because of |Vthb|>|Vtha|, there is a possibility that the drain current of the driving TFT 2903 is not 0. Even in case that the black display is carried out, there is such a risk that the drain current flows through the driving TFT 2903 and the EL element 2906 emits light, and there occurs a problem that contrast comes down.
Further, in the conventional display device of the current writing type, the video signal input current source for inputting the signal current to each pixel is disposed with respect to each column (with respect to each pixel line). There is a necessity that current characteristics of those all video signal input current sources are made to be the same and a current value to be outputted is analogously changed with accuracy. However, in a transistor using polycrystalline semiconductors etc., since variation of characteristics of transistors is large, it is difficult to make the video signal input current source in which current characteristics are uniform. Thus, in the conventional display device of the current writing type, the video signal input current source is fabricated on a single crystalline IC substrate. On the other hand, it is general that as to a substrate on which the pixel is formed, it is fabricated on an insulation substrate such as glass etc. from the aspect of cost etc. Then, there is a necessity that a single crystalline IC substrate on which the video signal input current source is fabricated is attached over a substrate over which the pixel is formed. The display device of such structure has such problems that cost is high, and an area of a picture frame can not be reduced since large is an area which is required on the occasion of attachment of the single crystalline IC substrate.
In view of the above-described actual condition, the invention has a task to provide a display device in which a light emitting element can be made to emit light with constant luminance without coming under the influence of deterioration over time and a driving method thereof. Also, the invention provides a display device in which it is possible to carry out accurate gray scale expression, and also, it is possible to speed up writing of a video signal to each pixel, and influence of noise such as a leak current etc. is suppressed and a driving method thereof. Furthermore, the invention has a task to provide a display device which reduces an area of a picture frame and realizes miniaturization and a driving method thereof.